assurance
n. C / Un. a promise or a strong statement that something is true or will happen. It can also mean the feeling of being confident in your own abilities.
n. a positive declaration intended to give confidence; a promise. Also refers to freedom from doubt or self-confidence in one's manner.
She gave me her personal assurance that the work would be finished.
The manager provided several assurances that no staff members would lose their jobs during the merger.
Despite his outward assurance and calm demeanor, the young lawyer felt a deep sense of trepidation as he approached the bench for the first time.
From Middle English assuraunce, from Old French asseürance, from asseürer; as if assure + -ance.
Often used with the verb 'give' or 'provide'. When meaning self-confidence, it is typically uncountable; when meaning a promise, it is often countable.